Eromanga Basin

Eromanga Basin location map

The Eromanga Basin is a Early Jurassic - Late Cretaceous basin.

The Eromanga Basin encloses the multi-aquifer system of the Great Artesian Basin and overlies late Palaeozoic and older basins. It consists of a broad downwarp with two main depocentres — the Poolowanna Trough and the Cooper region.

Summary

Age

Early Jurassic - Late Cretaceous

Area in South Australia

360 000 km2 (139 000 sq miles)

Exploration Well Density

1 well per 436 km2 (1 well per 168 sq. miles)

Success ratio

0.468

Depth to target zone

1200-3000m

Thickness

Up to 3000m

Hydrocarbon shows

Commercial discoveries of oil from almost every unit from the Poolowanna to the base Cadna-owie Formation in the Cooper region; elsewhere shows in the Poolowanna Formation.

The Eromanga Basin covers 1 000 000 km2 of central–eastern Australia, 360 000 km2 of which lie in South Australia. The Eromanga Basin encloses the multi-aquifer system of the Great Artesian Basin.

In South Australia, the Eromanga Basin overlies late Palaeozoic and older basins. It consists of a broad downwarp with two main depocentres — the Poolowanna Trough and the Cooper region — containing up to 3000 m of sediment. The central Eromanga Basin is overlain by the Tertiary to Recent Lake Eyre Basin. Eromanga Basin units crop out extensively on the western and southern margins.

Structurally, the Eromanga Basin is divided into two by the NE-trending Birdsville Track Ridge, a complex of related domes and ridges. The Poolowanna Trough in the NW contains a thick sand-dominated sequence in comparison to the Cooper region where intercalated shale and siltstone units occur.

Petroleum exploration commenced in the 1950s when licences covering the Cooper and Eromanga basins were first acquired by Santos, who went against conventional wisdom that commercial accumulations of oil would not be found in Mesozoic formations within the Great Artesian Basin.

Initial exploration involved surface mapping, stratigraphic drilling, aerial surveys, gravity and aeromagnetic surveys and seismic. The first petroleum well was drilled in 1959 and Cooper Basin gas was discovered in 1963.

The first commercial hydrocarbon to flow from the Eromanga Basin was gas produced from Namur 1 in 1976 (Cooper region). Oil was discovered in 1977 with an uneconomic flow from Poolowanna 1 in the Poolowanna Trough. The first economic oil flow was recorded from Strzelecki 3 (Cooper region) in the following year and this stimulated a major oil exploration program.

Since 1959 over 2000 wells have penetrated the Eromanga Basin sequence and over 100 000 km of seismic has been acquired. Exploration has concentrated in the Cooper region. A new phase of exploration for oil in the Eromanga Basin commenced in 2002 in the 27 new licences resulting from the expiry of PELs 5 and 6 in 1999. Most new entrant explorers are currently targeting Eromanga Basin oil plays.

In the Poolowanna Trough, the Poolowanna Formation (up to 130 m thick) is overlain by a thick sand-dominated unit (Algebuckina Sandstone). West of the northern Birdsville Track Ridge, Birkhead and Murta Formation shales pinch out into Algebuckina Sandstone. Algebuckina Sandstone crops out on the western and southern basin margin.

The non-marine sequence is succeeded conformably by a sequence reflecting transition from non-marine to marginal marine to open marine shale and sandstone. The basal unit, Cadna-owie Formation, is of significance for petroleum exploration as the top of the unit approximates a distinctive seismic reflector — the C horizon mappable over the entire basin. DSD (formerly DMITRE) has compiled a basin-wide C horizon data set from company seismic maps. The C horizon mapping now extends into Queensland, Northern Territory and New South Wales as a result of the National Geoscience Mapping Accord Cooper and Eromanga Basin project.

The upper non-marine sequence (Winton Formation) was rapidly deposited — up to 1100 m over ~8 million years. A period of erosion in the Late Cretaceous, caused by a switch in drainage from the Cooper region to the Ceduna Depocentre on the rifted southern margin, was followed by deposition of the non-marine Cainozoic Lake Eyre Basin.

Vertical migration of oil from Permian (Cooper Basin) source rocks has been widely accepted as the principal source of most Eromanga-reservoired oil (in the Cooper region). Both Cooper and Eromanga mature source rocks have contributed to oil accumulations in the region, however each oil accumulation needs to be considered on its merits with respect to the extent of mixing from Permian and Mesozoic sources. The Poolowanna and Birkhead formations contain organic-rich shales that are oil-prone and in places at peak maturity for oil generation. Lateral migration from these source areas has also been postulated.

Elsewhere in the basin, the presence of thick Poolowanna, Birkhead and Murta formations is critical to evaluation of oil source potential. The marine sequence and upper non-marine sequence are immature for hydrocarbon generation over much of the basin. The underlying Simpson and Pedirka basins also contain mature source rocks and are well placed to charge Eromanga Basin reservoirs.

Principal reservoirs in the Cooper region are the braided fluvial Hutton and Namur sandstones (porosities up to 25%, permeability up to 2500 mD). Oil is also reservoired in meandering fluvial (Poolowanna and Birkhead formations), lacustrine shoreface (McKinlay Member) and lacustrine turbidite (Murta Formation) sandstones. Detailed petrophysical data is available from DMITRE for all of these Eromanga Basin reservoirs. A schematic section showing typical petroleum traps of the Eromanga Basin.

In the Cooper region the structural framework of the Eromanga Basin is largely inherited from mild but widespread compression, regional tilt and erosion in the Late Triassic which produced the Nappamerri unconformity surface (N seismic horizon). Regional downwarping commenced in the Early Jurassic. Across the basin, Tertiary W–E compression reactivated Palaeozoic structures.

Seals consist of intraformational diagenetic sandstones, siltstones and shales of the Poolowanna, Birkhead and Murta formations in the Cooper region. In the Poolowanna Trough, they consist of intraformational siltstone and shale in the Poolowanna Formation and siltstone of the Cadna-owie Formation. Elsewhere in the basin, potential seals include the Cadna-owie Formation and Bulldog Shale – Wallumbilla Formation).

In the Cooper region, where the Nappamerri Group regional seal is thin or absent, oil and gas pools are found in coaxial Permian–Mesozoic structures (in some cases from the Patchawarra to Murta formations).

Trapping mechanisms within the Eromanga Basin are dominantly structural (anticlines with four-way dip closure or drapes over pre-existing highs) with a stratigraphic component (e.g. Hutton–Birkhead transition, Poolowanna, McKinlay Member and Murta Formation). Eromanga structures in South Australia are rarely filled to spill with oil — net oil columns are relatively thin compared to the height under closure (due to poor sealing characteristics).

Cooper Basin region of South Australia

Petroleum exploration in the Eromanga Basin in South Australia has traditionally concentrated in the portion underlain by the highly productive Cooper Basin, a relatively mature area. In addition, there is the likelihood that a significant amount of the oil found in this area has been sourced from the underlying Cooper Basin.

The USGS (schenk et al 2016) has recently estimated the undiscovered conventional oil and gas potential of the Eromanga Basin in this area of South Australia. Table 1 summarises the results of this assessment. Further potential has also been assessed for the underlying Cooper Basin.

Western Eromanga Basin

Areas to the west have had minimal exploration effort, with only one sub-economic discovery. Oil found here would probably be sourced from the Poolowanna and Birkhead formations or the underlying Pedirka and Simpson basins. Estimates of undiscovered resources in the western Eromanga are best carried out by a method that uses available geological data and Monte Carlo type statistical techniques to calculate, as a probability distribution, the undiscovered resources for each play.

For a commercial petroleum field to exist in the western Eromanga Basin, four essential components are required: a mature ‘source’, a ‘reservoir’ horizon, a ‘seal’ horizon and a structure over the reservoir horizon that will concentrate the petroleum in economic quantities and that was present at the time of the petroleum expulsion from the source rock. Usually this is an anticline, but stratigraphic traps can also be important. When all four of these occur together, a petroleum ‘play’ or a potential target for exploration exists.

Maps for each play, taking into account distribution of source, seal and reservoir were constructed. Figure 7 summarise these for the Poolowanna Formation, Hutton Sandstone and Namur–Algebuckina sandstones plays respectively.

Table 1 summarises the results of the assessment of the undiscovered resources of the western Eromanga Basin at various probability levels.

Exploration is permitted in the Simpson Desert Regional Reserve, the Innamincka Regional Reserve, the Witjira National Park, Tallaringa Conservation Park and the Lake Eyre National Park. Exploration is not permitted in the Coongie Lakes National Park, the Simpson Desert Conservation Park or the No-Go Special Management Zone of the Innamincka Regional Reserve.

The initial 2001 right to negotiate (RTN) agreements in the Cooper Basin for the CO-98 PEL application areas were groundbreaking in Australia as the first conjunctive agreements (covering exploration and production) that provide certainty in enabling any explorer, the Aboriginal parties, and the state being able to benefit from any commercial discoveries made.

The state has also commenced the RTN process for additional PELs in the Eromanga and Arrowie basins and it is likely that existing RTN agreements will continue to form a practical precedent. Any agreements will provide for protection of the heritage and cultural interests of the Aboriginal parties.

Compulsory relinquishment of roughly 36% (19 150 km2) of the areas in current Cooper region PELs will precede competitive bidding from 2009. In anticipation of the calling for work program bids in 2009, negotiations opened in 2006 to develop a conjunctive Indigenous land access agreement (ILUA) for the regions already covered with land access agreements resulting from earlier RTN proceedings. These negotiations involve the Aboriginal Legal Rights Movement, the three native title parties already familiar with the RTN process, the South Australian Government, and petroleum exploration and production company representatives (through the South Australian Chamber of Mines and Energy).

The draft framework agreement established in 2006 will be the subject of necessary formal negotiations as required by the Native Title Act 1993, with a hope to formalise ILUAs for the whole of the Cooper Basin region in 2007.

The first conjunctive ILUA in Australia in a proven petroleum province was concluded in February 2007 with the Yandruwandha–Yawarrawarrka native title claimants over a major portion of the South Australian Cooper Basin (Fig. 1, Aboriginal issues in the ‘Land access’ section of this USB). This will provide greater certainty and expedite the grant of PELs in a way that remains fair to native title claimants and sustainable in relation to exploration and production investment. The application of conjunctive ILUAs will enable land access more quickly and with lower transaction costs than serial RTN proceedings. The successful implementation of conjunctive ILUAs for Cooper Basin petroleum exploration and production will serve as a model for analogous agreements elsewhere in the state.

Negotiations in respect to formalising conjunctive petroleum ILUAs with the remaining two native title claimant parties in the Cooper Basin are progressing.

In summary, conjunctive ILUAs are proposed as an evolutionary, additional, alternative to the RTN process already working comparatively well in South Australia. Indeed, conjunctive ILUAs will be an attractive incentive to achieve competitive bids, with explorers knowing the terms of land access prior to lodging bids.

A number of applications are being held over the western part of the Eromanga Basin pending resolution of native title issues.

Sales et al (2015) has concluded that best prospects for new significant oil discoveries are on the western flank of the Cooper Basin. Two plays are evident and may be subtle. The dominant play is in the structurally subtle Namur Sandstone, while the secondary is the mid-Birkhead Formation which is stratigraphically subtle and may be difficult to delineate, depositionally complex and a challenge to produce.

The recent Kangaroo-1 exploration well drilled in September 2016 intersected a 20 metre gross oil column in the Birkhead Formation with a calculated oil flowrate of 320 bopd. The oil discovery in Kangaroo-1 have expanded the prospectivity of the north western flank further west than previous exploration, highlighting significant under explored areas.

Licence activity in the previous year is detailed in the ‘Exploration and development’ section of this USB and Figure 8 shows the licence status at the time of publication. Use this link for further information on holders of petroleum tenements in South Australia.